Perfluoroalkyl anions are extremely unstable. Early attempts to generate nucleophilic metal trifluoromethide compounds were unsuccessful. See Haszeldine, J. Chem. Soc., 1954, 1273; Pierce et al, J. Am. Chem. Soc., 1954, 76, 474; Bergman, J. Org. Chem., 1958, 23, 476; McBee et al, J. Org. Chem., 1963, 28, 1131. Presumably, this is due to the rapid dissociation of CF.sub.3 into CF.sub.2 and F.sup.- (Hine, Physical Organic Chemistry, McGraw-Hill, New York, 1962, p 486). More recently, reports have appeared describing trifluoromethylcopper (CF.sub.3 Cu), which exhibits nucleophilic character--see Wiemers et al, J. Am. Chem. Soc., 1986, 108, 832 and references cited therein; and McClinton et al, Chem. Commun., 1988, 638. This reagent will displace halogen atoms from aromatic rings. One method of CF.sub.3 Cu generation is the decomposition of metal trifluoroacetates in the presence of copper (I) iodide (Matsui et al, Chem. Letters, 1981, 1719; Carr et al, J. Chem. Soc., Perkin Trans. 2, 1988, 921).